A.1. Field of the Invention
The invention relates in its generality to a system for the transmission of a video signal from a transmitter to a receiver, and more specifically to a system for the transmission of a video signal in a digitized form obtained by means of differential pulse-code modulation (DPCM), the composite signal words of the digitized video signal additionally being submitted to a variable-length coding operation.
The invention also relates to a transmitter and a receiver suitable for the system.
A.2. Description of the Prior Art
Digitizing analog information signals has of late received an increasing interest. Relative to analog signals, digital signals generally have the advantage that noise superimposed on them during the transmission can be removed. The bit rate of the digital signal is highly dependent on the type of analog signal; that is to say its bandwidth and the number of bits required to represent each sample of the analog signal so as to obtain a predetermined quality. Thus, a digitized TV-image needs approximately 5 Mbits, which corresponds to a bit rate of approximately 120 Mbit per second. For the majority of transmission media this bit rate is impermissibly high. For that reason a number of techniques have been developed to reduce this bit rate. These techniques have for their object to reduce either the number of bits per sample or the number of samples to be transmitted, or a combination of both techniques.
An example of a technique which reduces the number of samples to be transmitted is the transformation coding technique. Therein an image is divided into sub-images which are each considered to be a sum of a plurality of mutually orthogonal basic images, each having its own weight factor. Only a limited number of these weight factors are encoded for transmission to the receiver, for example only the weight factors having an absolute value which exceeds a threshold value. In this case the transmitted weight factors constitute the signal words mentioned in the foregoing.
A bit rate reduction technique which has for its object to reduce the number of bits per sample is the differential pulse code modulation (DPCM). Therein a prediction signal sample is always subtracted in the transmitter from a video signal sample to generate a differential signal sample which is quantized and encoded. The differential signal samples thus quantized and encoded represent the above-mentioned signal words. They are transmitted via a transmission medium to the receiver where they are converted into the original video signal samples by means of a circuit having integrating properties. The bit rate reduction is realized by the fact that the differential signal samples are generally distributed over a smaller dynamic range than the original video signal samples and consequently can be represented by a lower number of bits.
A known technique of realizing a further bit rate reduction for any digital signal consisting of signal words is the variable-length coding. Therein a variable-length coding book is defined and one code word from this coding book is assigned to each signal word. More specifically, a shorter code word is assigned to a signal word which statistically occurs often than to a signal word which statistically occurs less often. The structure of the variable-length coding book and the allocation depends on the type of signal. After transmission, the code words are reconverted into the original signal words.
Such a variable-length coding can be applied to the quantized and encoded difference signal samples obtained by differential pulse code modulation (see Pratt: Image Transmission Techniques, Academic Press, New York 1979, page 16).